<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div {
            width: 0;
            height: 0;
            border: 50px solid transparent;
            /* 边分原理 */
            border-right-color: red;
        }
    </style>
</head>

<body>
    <div>

    </div>
</body>

</html>